Improved electrical potentiometer



Nov. 5, 1957 E. A. WILLIFORD 2,812,408

IMPROVED ELECTRICAL POTENTIOMETER Filed June 10, 1955 FIG. 50.,

United States Patent Ofiice IMPROVED ELECTRICAL POTENTIOMETER Edward A. Williford, Binghamton, N. Y., assignor to Link Aviation, Inc., Binghamton, N. Y., a corporation of New York Application June 10, 1955, Serial No. 514,568

4 Claims. (Cl. 201-56) This invention relates to improvements in variable potential dividers, and more particularly to improved resilient contact means for providing a reversibly variable contact which is completely free from backlash, which maintains constant contact pressure to avoid electrical noise, and which assures good electrical contact with a wire-wound conductive element while requiring a minimum of torque for its operation.

In the electronic instrumentation, communication and computation arts, high-precision variable resistance potentiometers are employed to derive electrical potentials which vary in accordance with predetermined mathematical functions. As employed in analog computers, a plurality of such potentiometers are frequently coupled to common drive shafts and operated by reversible electric motors, or sometimes by selsyns, to derive voltages representing functions of angular shaft rotation. Where a number of such potentiometers ganged to a common shaft may be required to be operated by a small servo or selsyn motor, it is necessary that the friction of the moving parts in the potentiometers be held to a minimum in order that the low torque of the servo or selsyn may accurately position the contact wipers. It is also necessary that uniform electrical contact be maintained between the resistance element of each potentiometer and its movable wiper to avoid generation of electrical noise.

To meet all of these requirements, and to keep the movable contactor mechanism entirely within the cylindrical shell of the potentiometer resistance element, poten tiometers of the prior art have commonly employed a resilient cantilever arm comprised of, or at least plated with, precious metal at the point of contact. It has been discovered, however, that such forms of construction invariably cause different forces to be exerted on the wiper element when rotated in opposite directions, resulting in sliding of the wiper when it is rotated in one direction and stubbing of the wiper when rotated in the opposite direction. This leads to such undesirable functioning as unequal torque in opposite directions of rotation, and a degree of electrical backlash which may introduce serious errors in an electrical computer. Stubbing or vibration of the contact alters contact pressure and contact resistance, causing high frequency noise in many circuits. When a plurality of such prior art potentiometers are connected in cascade, as in a computer system, small positional errors due to this backlash in the cantilever contactors are additive and may produce much larger errors in the output circuits of the system, and noise introduced by a prior art potentiometer in one portion of a computer system may be greatly amplified in the computer so as to cause faulty operation.

It is therefore an object of the invention to provide a precision variable potentiometer having virtually no backlash in the movable contact member and having substantially uniform contact resistance and pressure regardless of direction of operation.

Another object of the invention is to provide improved 2,812,408 Patented Nov. 5, 1957 low torque variable potentiometers suitable for ganged operation from a common drive means.

Another object of the invention is to provide a precision variable potentiometer reversibly operable in either direction by the application of equal torque or force without the introduction of variable functional performance.

A further object of the invention is to provide such improved precision potentiometers with means for readily adjusting the contact pressure between the movable elements and the stationary conductors.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawing, in which:

Fig. 1 represents a conventional potentiometer of the prior art;

Fig. 2 represents a perspective view of an improved potentiometer embodying the invention;

Fig. 3 is an enlarged view in perspective of the contact wiper element of the potentiometer shown in Fig. 2, embodying features of the invention;

Fig. 4 illustrates in perspective an alternative embodiment of adjustable contact wiper element suitable for use with the potentiometer shown in Fig. 2;

Fig. 5 is a detailed side view in section of the structure shown in Fig. 4;

Fig. 5a is a perspective view showing in detail one form of compression spring suitable for use in the structure of Fig. 4 and Fig. 5;

Fig. 6 is a perspective view of an alternative arrangement which may be used in adjusting contact pressure.

Referring first to the prior art type of potentiometer illustrated by Fig. l of the drawings, a cylindrical card of insulating material 7 has wound upon it a multiplicity of evenly spaced coils of fine resistance wire, forming a helical conductive element 8. Although other forms of conductive element may be employed in some types of potentiometers, as for example a coating of carbonized material, high precision potentiometers as used in electrical computers generally require a resistance element of metallic wire. A rotatable shaft 9, which is coaxial with the helix 8, is coupled to a rigid arm 10 which is rotatable within the cylindrical enclosure of the helix 8. Arm 10 carries a resilient contact or wiper element 11 formed as a cantilever, which may be secured to arm 10 by soldering, riveting or by means of a set screw 12. An arcuate portion 13 of the wiper element 11 bears against a portion of the inner surface of the wire helix 8, being urged into conductive engagement with the resistive element 8 by the spring tension of cantilever 11.

Numerous variations of this basic construction have been employed in precision potentiometers of the prior art, but all known prior forms of wiper construction result in different forces appearing to be exerted on the wiper 11 for different directions of rotation. As viewed in Fig. l, the wiper arm 11 may deflect slightly to the left as the shaft 9 is rotated in a clockwise direction and dragging the arm 11, and in general the arm 11 will deflect an even greater distance to the right when the shaft 9 is rotated in a counter-clockwise direction, pushing the wiper ahead of the arm and causing it to stub, Any deflection in either direction results in failure of the contact portion 13 to accurately follow the angular rotation of shaft 9, and unequal deflectionin opposite direcmeter 30 are fastened tions of rotation results in more than doubling the positional errors'beiwe'enthe shaft 9 "and "the contact portion 13 as the direction of rotation is reversed. Such errors may become cumulative in-complex systemswhere a plurality of potentiometers are connected in cascade,'and they impose an undesirable limitation to the accuracy which has heretofore been'possible in electrical analog computer systems.

Reference is nowhad to Fig. and Fig. 3 of the drawings which disclose one embodiment of my invention "whereby errors inpotentiometer wiper deflection are obviated. A substantially cylindrical casing encloses a wirewound resistance card 21 which may be held in position within the casingjll by anysuitable means. A cenrain disposed coaxial shaft 22 is channeled through a bearing (not shown li e fendmn of't he casing 20. A "rigid aria inember 23, I, hichma'y contain a coaxial ring e la 24 "for engagingtheshaft Z'Z, isclamped on to the rotatable shaft 22by means a clamping screw 25. The opposite 'end'o f ng'idarin 22 may carry a fixed pin 26 adapted to engage a-cooperat'ive "socket of an adjacent p"'"e' ntiornter whentwofor'mor'e potentiometers are coaxlally ganged for simultaneous operation by a single rotatable' s'haft. The end 27 er-arm 2 2 is polyangular in cross-section and carries a slidable collar member 28 "which maybe secured to'tlie end '27 by means of a set screw 29. Affixed to opposite sides of collar 28 is a resilientspring contact'o'r element 30 having a 'tip portion 31 which maybe of precious metal and which is in conductive engagement with the inner cylindrical wall of the re'sista'nc'eelement 21. A'sshaft 22 is rotated in either direction, theconductiv'etip perm-131 is moved through an arcfof 'a'cir'cle, contacting c'drresponding portions of the resistance 'elernentateachdifferent position to which the shaft 22m-a'y be rotated.

Although the rotatable contactorarm 23 is normally fixed to a rotatable shaft 22, whereby rotational motion is jimparted to the contactor "tip portion 31, in some em- "bodiinentsof theinvention it"in'ay*b'e desirable to rotatably mount the arm 23 on a fixed concentric shaft 22, and "to impart "rotational motion'to "the contactor arm 23 by 's'ni'tab l'e coupling" he pin 26. In such arrangements thecollarl t m'ayb of'bearing metal adapted to engage fan annular groove in' t'heflfixed rotational shaft 22. This form of "construction may lie patticula'rlyuseful in potentidmeters intended tbbe 'coaxially ganged fonmultiple e eratien. n I n An annular slipfr'in'g'3" "of conau nvematenn is mountedo'n the end of 'thefia s'ing 20, withinfthe cylinder "formed by'the-resis'ta j21 fand is electrically connected to an axe na l terminal "35. A brush wiper 3 6, *wliich 'may be "f ed'ojf *Pil'os'phor bronze 'orother suitable spring "mater ,is fastened to the slidable collar 28 and Wipes upon the' ann'ular ingatt maintain electrical contact "at "an times "betwee "the fixed external terminal 35 "and the "movable spring 106p fcdntaetor 30. External terminals 37 and a're' conriect'ed to oppositee'nds of'the resistance "element 21. Aiditidna1 tenninals (not shown may also be provided"for'connections tofixe'dtaps (not shown) which 'nray'be made to va'rious portions of the resistance element 21.

V The spring 'conta'ctor 30 inay be formed of thin strips of Phosphor bronze, or other "suitable "spring material, "or 'this elementmay' fol-me of springwir'e which'is substand-any circular ln *crtn's-seetits Ks illustrated in greater detai'l'inFig. 3, th'ebppo'site ends of spring con- 7 to the slidablecollar member 28 by ;rtvet in s61dering o spcrw rding, as shown at 40in 'Fi g. 3. The s'firiii'QbohtfiCtbi 30 is formed substantially as anilongateddoopwi'th its-major axis extending in a direction at right times "tithe 'axi'sof shaft 22 and its irder laceration "a r'adial 'lin'e trbmfaie axis or shaft 122 through in .tsantbnfsi '61: the warmer so, which ts are fingernail ""ritae nsjdp in engagement with-ate elefiinfzl. "crooaetec'trtcal eeiita'ctnetvv'eeh 4 the contactor tip portion 31 and the conductive element 21 is as'sufe'dby compressional stress on the arms of loop 36 Any desired degree of'compression of loop 30 may be obtained by loosening set screw 29 and sliding collar 28 radially along the end 27 of rotatable arm 23.

Reference is now had to Figs. 4 and 5 of the drawings which disclose an alternative embodiment of the invention wherein one end-27 of the rotatable arm 23 may be capped by a polyangular channel-like member 44, and thespring loop contactor 30' is affixed to a slidable block 42 within the hollow portion of the channel 44. In Fig. 4, the contactor loop 30' is formed entirely of spring wire having a substantially circular cross-section, while an arcuate contactor tip portion 31' on the minor axis of loop 30' may be coated withprecious metal to assure good electrical contact with the resistance element 21.

The lower surface 45 of the slidable block 42 forms an angular cam, as clearly shown in Fig. 5, and the set-screw 29 is threaded through a side wall of the hollow channel 44in a position to engage the inclined cam surface 45 of block 42. As the-set screw 29 of Fig. 4 and Fig. 5 is tightened, it exerts a force upon the inclined cam surface 45 of block 42 causing the block 42 and the spring loop 30' to move radially outward, compressing the spring loop 39' and increasing contact pressure between the resistance element 21 and the contact tip portion 31'. Conversely, as the set screw 29 is loosened, block 42 is allowed to move inwardly, within the confines of channel arm '44, thereby reducing compressional stress on loop spring 30, and correspondingly reducing contact pressure between the tip portion 31' and the resistance coil 21. If desired, a portion of the outer end of channel '44 may be'folded over as shown at 47, and a compression spring 46 may be placed between the outer end of slidable block 42 and the folded end 47, to urge the spring loop 38' inwardly as set screw 29 is loosened. By this means the contact pressure between contact tip portion 31' and the conductive element 21 may be adjusted to a very low value and still assure positive contact free from chatter as the wiper arm is rotated. A form of compression spring suitable for this purpose is illustrated by Fig. 5:1. It will be understood, however, that either a leaf spring, a helical spring, or any other suitable form of compression spring may be' employed at 46 if desired.

Shown in Fig. 6 is an alternative embodiment of the invention utilizing different means to vary contact pressure. In Fig. 6 the wiper loop is rigidly affixed at each end to the end portion 27 of rotatable arm 23 by means of welding, soldering, or by means of rivets or screws 40. Nearer the end of the rigid arm, each side of the loop is 'adjustably compressed, as for example by means of adjustablescrews 29a and 2917, which thread'into the end of the rigid arm. In Fig. 6 the resilient contact loop is shown in full lines in an uncompressed condition, and in dashed lines in-a compressed condition. As screws 2921 and 2% are threaded into the rigid arm, the contact loop is resiliently deformed, with the major axis of the loop decreasing and the minor axis increasing. As "will be "evident, such deformation urges contact tip portion 31 radially outward, increasing the pressure between tip 31 and 'a potentiometer resistance element. It will be seen that independent adjustment of screw 29a or of screw 29b would serve to tilt or cock the loop. For example, tightening of screw 2% without an equal tightening of screw 29a'would'ser've to'rotate the axes of the wiper-loop .c'ounter clock'wise as viewed in Fig. 6. In practice of the -tac tor element which is continually'urider controlled compfess ional'str'e's's in engagement with the conductive resistance element. As opposite ends of the contactor loop element 30 are securely anchored to a rigid rotatable arm, virtually no angular deflection or backlash can occur between the rigid arm and the contacting portion 31 of the contactor tip element, regardless of the direction of shaft rotation. It will also be appreciated that as the friction between the movable contactor tip portion 31 and the resistance element 21 is dependent entirely upon the compressional stress applied to spring loop 30, and is wholly unafiected by direction of rotation, the potentiometer may be operated in either direction by application of equal values of minimum torque. The contact pressure may be readily adjusted by means of set screw 29 to obtain good electrical contact with a minimum of friction between the contactor tip 31 and the conductive element 21.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is particularly noted that although the invention has been disclosed as embodied in a variable potentiometer of the wire-wound resistive type, the structure of the invention is equally applicable to variable inductive apparatus, as for example variable auto-transformers, or to other forms of variable potential dividers, or wherever it is desired to obtain a low torque resilient contact free from angular backlash. Furthermore, while I have shown the invention as applied to a rotary potentiometer, the novel wiper contact of the invention may also be applied to linearly-actuated potentiometers.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In variable potential divider apparatus having an annular conductive element and a rotational shaft concentric therewith, low-torque variable contact means comprising a resilient spring contact member substantially formed in an elongated loop, a conductive portion of said contact member on the minor axis of said loop and in electrical contact with the conductive element, the minor axis of said loop substantially normal to the point of contact with the conductive element, a rigid arm mounted on the rotational shaft, a movable member slidably mounted on said arm, means fastening opposite ends of said spring contact member to said movable member, said fastening means positioning the loop of said spring contact member symmetrically about a radial line extending from the axis of the rotational shaft to the point of contact between said conductive portion and the conductive element, and adjustable means for fixing the position of said movable member on said arm to vary the compressional tension of said spring contact member.

2. In variable potential divider apparatus having a conductive element of substantially cylindrical form and a rotational shaft concentric therewith, low-torque variable contact means comprising, a resilient spring contact member substantially formed in an elongated loop, the minor axis of said loop substantially normal to the inner cylinder wall of the conductive element, a conductive portion of said contact member on said minor axis and in electrical contact with the inner wall of the conductive element, means fastening opposite ends of said spring contact member to a movable member, said fastening means positioning the loop of said spring contact member symmetrically about the minor axis thereof, a rigid arm member extending radially within the cylinder of the conductive element, means on said rigid arm member slidably engaging said movable member, a set screw on said arm member for adjustably fixing the position of said movable member with respect to said arm, further spring means between said arm and said movable member, said further spring means cooperating with said set screw to determine the radial position of said movable member on said arm, and means mounting said arm on the rotational shaft of the potential divider apparatus whereby said arm and said contact member may be rotated as a unit.

3. A rotatable electrical contact engageable with the cylinder wall of annularly disposed conductive elements comprising, a shaft concentric with the conductive elements, a rigid arm rotatably mounted on said shaft and extending radially therefrom within the cylinder formed by the elements, one end of said arm of polyangular crosssection, a polyangular collar slidably mounted on said end, a set screw on said collar for adjustably fixing the position thereof on said ram, a resilient spring member formed substantially in a loop, opposite ends of said loop rigidly fastened to opposite sides of said collar, and a mid-portion of said spring loop adapted to conductively engage the inner cylindrical surface of said annularly disposed elements.

4. A rotatable electrical contact engageable with a plurality of annularly disposed conductive elements comprising, a shaft concentric with the conductive elements, a rigid arm rotatably mounted on said shaft and extending radially therefrom in a direction toward the conductive elements, one end of said rigid arm comprising a polyangular channel portion, a polyangular member slidably mounted within said channel portion, one end of said polyangular member comprising an inclined cam surface, a resilient spring member formed substantially in a loop, opposite ends of said loop fastened to opposite edges of said polyangular member at one end thereof, a midportion of said spring loop adapted to conductively engage successive conductive elements as said arm is rotated, and a set screw mounted on said channel portion and engaged with said cam surface whereby the compression of said spring loop may be varied to adjust the contact pressure of said mid-portion on the conductive elements.

References Cited in the tile of this patent UNITED STATES PATENTS 

